To reduce the radical, you have to factorize 108.
108 is a multiple of 3, so to factorize it, you can divide it by 3
[tex]\frac{108}{3}=36[/tex]You can rewrite the square root as:
[tex]\sqrt[]{3\cdot36}=\sqrt[]{3}\cdot\sqrt[]{36}[/tex]The square root of 36 is equal to 6 so you can write the expression as:
[tex]\sqrt[]{3}\cdot\sqrt[]{36}=6\sqrt[]{3}[/tex]